The easiest to change out is the furniture and HTS. That will give you 6 of the required 7 parts. The gas piston, muzzle device and operating rod (charging handle) are also easy to replace.
There are also followers and base plates available, but I've never liked using mag parts to make my count.
This is the best US made HTS set available right now.
Tapco, gunpartsguy, DSA, Falcon arms and gunthings all carry compliance parts.